1. Field of the Invention
This invention relates to apparatus for producing an optical fiber. More particularly, it relates to apparatus for producing an optical fiber wherein an optical fiber preform (hereinbelow, simply termed "preform") is heated by a heater arrangement, and one end of the preform in the molten state is drawn so as to obtain an optical fiber of uniform predetermined diameter.
2. Brief Description of the Prior Art
For an optical fiber which is used as a transmission line of optical signals in an optical communication system, a severe uniformity in the fiber diameter is required in order to render the transmission loss ultralow.
The uniformity in the diameter of an optical fiber depends on a drawing machine for the optical fiber. Heretofore, the preform process has been mainly adopted for the drawing of the ultralow loss optical fiber, and an apparatus as shown in FIG. 1 has been employed. The process uses as an optical fiber material a pipe or rod, i.e. a preform which is separately prepared and which is made of one or more layers of vitreous material. The preform 1 (having an outside diameter D) is inserted at a fixed speed v.sub.p into a protection tube 3 in a furnace heated by a heating source 2. The lower end part of the preform heated and molten is drawn out, and is taken up round a drum 6. A motor (not shown) is driven while being controlled by a motor controller 8. Thus, the drum 6 is rotated and winds an optical fiber 9 thereon while elongating it at a fixed speed v.sub.f. In this way, the optical fiber 9 having a predetermined outside diameter d is formed. The diameter d of the optical fiber 9 is detected by an optical non-contact type fiber detector 4, and is indicated on a fiber diameter measuring device 5. In the presence of a fluctuation in the diameter, the fiber diameter control is carried out in such a way that an analog output of the fiber diameter measuring device 5 is fed to a fiber diameter controlling circuit 7 wherein it is processed by comparison with a signal corresponding to a set diameter value to produce an output signal of the control circuit which is fed to the motor controller 8 to change the rotational speed of the drum 6 (corresponding to changing the speed v.sub.f). With the fiber diameter controlling method, the fiber diameter fluctuations amounted to .+-. several % and it was difficult to suppress the fluctuations below the values.
The prior art method was studied by Katsuyuki Imoto et al from various viewpoints, as well as investigating the mechanism of fiber diameter fluctuations. As the result, it was revealed that factors for the fiber diameter fluctuations are broadly classified into the following two types: (1) structural imperfections of the preform (variations in the outside diameter, offset of the axis, inclination of the axis, etc.) and (2) fluctuations in the preform melting temperature during the drawing as are attributed to disturbances based on the factor (1) (changes in air current A flowing within the protection tube) and disturbances based on external factors (changes in air current A flowing within the protection tube).
Fiber diameter fluctuations due to the factor (2) often occurred during the drawing, and they amounted to .+-. several % to .+-. several tens %. Moreover, the time constant at the fiber diameter fluctuations was in the order of second, so it was revealed that the suppression of the fiber diameter fluctuations to below .+-. 2 to .+-. 3% is difficult even by changing the take-up speed v.sub.f. Katsuyuki Imoto et al have therefore previously proposed an optical fiber drawing machine which can suppress the fiber diameter fluctuations due to the factor (2) (Japanese Patent Application No. 142055/1975) and apparatuses for controlling the diameter of an optical fiber which control the fiber diameter so as to reduce the fiber diameter fluctuations due to the factor (1) while suppressing the fiber diameter fluctuations due to the factor (2) (Japanese Patent Applications Nos. 151825/1975 and 29960/1976). U.S. patent application Ser. No. 744,454 and West German Patent Application P 26 53 836.3-45 which correspond to these Japanese patent applications were respectively filed Nov. 23, 1976 and Nov. 26, 1976.
Examples of the apparatus for controlling the fiber diameter are shown in FIG. 2a and FIG. 2b. Referring to the figures, numeral 10 (10') designates a gas conduit (gas curtain device) which is mounted on the upper part (lower part) of a protection tube 3. The gas curtain device controls the flow rate of a fed and introduced gas 13 (14) by a valve regulator 12 (12') and forces the gas to flow out into the protection tube so as to suppress air current A flowing into the protection tube from the exterior. While thus suppressing the fiber diameter fluctuations due to the factor (2) as far as possible, the gas curtain device controls the fiber diameter fluctuations due to the factor (1) by changing the flow rate of the gas, to reduce the fiber diameter fluctuations. More specifically, it is the same as in the case of FIG. 1 that the fiber diameter fluctuation is detected by a fiber diameter detector 4 and that an analog output (digital output) from a fiber diameter measuring device 5 is fed into a fiber diameter controlling circuit 7 (provided that the construction of the control circuit differs). It is different from the case of the prior art apparatus in that an output signal from the fiber diameter controlling circuit 7 is utilized to control a valve regulator 12 (12') for controlling the gas flow rate.
In the figures, numeral 11 (11') denotes a gas flow meter. Numerals 13', 13", 14' and 14" indicate streams of the gas.
In the figures, the same symbols represent the same or functionally equivalent components such that additional descriptions are not provided.
In the case of FIG. 2b, there are two aspects; one aspect in which the output of the control circuit 7 is fed back to the valve regulator 12 (solid line) for controlling the gas flow rate, and the other aspect in which it is fed back to the valve regulator 12' (dotted line) for controlling the gas flow rate. According to this control method, usually one gas flow rate is fixed when the other gas flow rate is being controlled.
It was experimentally confirmed that, with the apparatus in FIG. 2a or FIG. 2b, even when the outside diameter variations of the preform are approximately .+-. 2%, the fiber diameter fluctuations of the optical fiber can be very stably controlled to approximately .+-. 1%. The apparatus accordingly proved to be an extraordinarily effective control mechanism. However, in the case where the outside diameter of the preform varies taperingly in the lengthwise direction thereof as shown in FIG. 3a, where the outside diameter is extremely large at a part in the lengthwise direction as shown in FIG. 3b, where the outside diameter is small at a part as shown in FIG. 3c, or further, in the case where a desired fiber diameter value to be initially set in the fiber diameter control circuit 7 is erroneous, a phenomenon in which the gas flow rate deviates sharply from the initially set value arises when the fiber diameter control is being conducted by changing the value of the gas flow rate. As the result, an influence is exerted on the temperature in the protection tube, to change the temperature of the molten part of the preform during the drawing of the optical fiber or to change a tension acting on the optical fiber at the drawing. This leads to the difficulties that the optical fiber comes to have inhomogeneous characteristics in the lengthwise direction thereof and that the fiber diameter control becomes impossible in extreme cases.